1. Field of the Invention
This invention relates generally to a hydraulic control system for an automatic transmission, and, more particularly, to a regulator valve and system for controlling pressure in a friction control element of the transmission.
2. Description of the Prior Art
A friction control element, such as a clutch or brake of an automatic transmission, can be applied and released using a combination of a regulator valve, latch valve and variable bleed solenoid (VBS) pressure in the hydraulic control circuit of the transmission.
Ideally, the control system applies the clutch by an increase VBS pressure, which opens the regulator valve and allows flow to the clutch from a source of line pressure. After application of the clutch is complete and at the end of the gear ratio change, the latch valve vents feedback pressure to the regulator valve, allowing full line pressure to be directly applied to the clutch.
The system releases the clutch by reducing VBS pressure until the latch valve moves to allow the feedback of the regulator valve to refill. Further lowering VBS pressure proportionally lowers clutch pressure by controlling the vent of clutch pressure to a pressurized exhaust.
Such control elements are susceptible to air ingestion, undershoot and sluggish response, and operational instability. Air ingestion occurs when the feedback pressure is vented during latching. Air remains in the chamber when the system is refilled, causing a high compliance, leading to overshoot, instability, and sluggish response of the regulator, and non-ideal control of clutch capacity. The overshoot in clutch pressure occurs during boost or stroke phase and low frequency shudder from loss of damping during ratio change phase.
An undershoot in clutch pressure occurs during the unlatch of an offgoing clutch event just before the gear ratio change, due to the dumping of feedback pressure to exhaust from an initial high line pressure. This adversely affects shift quality.
A change in the worm trail may prevent air ingestion or an increase in sump level, but achieving an ideal worm trail routing is not always feasible due to packaging requirements. Also, maintaining a high fluid level in the oil pan with a chain drive moving in the oil reduces fuel economy due to spin loss and potential aeration of oil.
Locating a regulator valve and latch valve in different layers of the main control, requires a large volume of worm trail between regulator outlet and feedback orifice, which decreases stiffness.